1. Field of the Invention
The present invention relates to zoom lenses suited to cameras for photography, video cameras and still video cameras.
2. Description of Related Art
A type of zoom lens in which the preceding lens unit is negative in refractive power, or the so-called negative lead type, is feasible for widening the angle of view with relative ease, so that the negative lead type zoom lens has found its use as the standard zoom lens in many cameras.
Of the standard lenses of the above type, there is a one which is constructed with a first lens unit of negative refractive power and a second lens unit of positive refractive power, totaling two lens units, the arrangement being made such that these two lens units move along a common optical axis in differential relation to vary the focal length, or the so-called 2-unit zoom lens, as, for example, proposed in Japanese Laid-Open Patent Applications No. Sho 53-132360 (corresponding to U.S. Pat. No. 4,299,452), No. Sho 56-19022 (corresponding to U.S. Pat. No. 4,370,031) and U.S. Pat. No 5,283,693.
For such 2-unit zoom lenses, the use of many aspheric surfaces reduces the number of constituent lenses to a compact form, as, for example, proposed in Japanese Laid-Open Patent Applications No. Hei 4-46308, No. Hei 4-46309, No. Hei 4-46310, No. Hei 4-56814, No. Hei 4-67112, No. Hei 4-67113 and No. Hei 9-33810.
Also, in Japanese Patent Publication No. Sho 60-46688 and Japanese Laid-Open Patent Application No. Hei 5-88084, a compact zoom lens is disclosed, in which the first lens unit is constructed with a negative lens and a positive lens, totaling two lenses, and the second lens unit is constructed with a positive lens, a positive lens, a negative lens and a positive lens, totaling four lenses. In Japanese Patent Publication No. Sho 61-42246, the first lens unit is constructed with a negative lens and a positive lens, totaling two lenses, and the second lens unit is constructed with four or five lenses.
In general, the negative lead type zoom lens comprising the first lens unit of negative refractive power and the second lens unit of positive refractive power not only has the advantage that the maximum field angle is relatively easy to increase, but also the advantage that a certain back focal distance is easy to obtain.
However, to simultaneously fulfill the requirements of making the entire lens system from as few lens elements as 4 to 8 and of obtaining a good optical performance, there is a need to appropriately determine the refractive power arrangement of all the lens elements in each unit, the forms of the lens elements and others. If these are inappropriate, the aberrations, during zooming, vary to a large extent, which cannot be remedied even if the number of lens elements is increased. Therefore, it becomes difficult to attain good stability of high optical performance throughout the entire zooming range.
For example, the zoom lens proposed in the above Japanese Laid-Open Patent Application No. Hei 9-33810, although its having a few lens elements, employs many aspheric surfaces. For this reason, the manufacturing tolerances become very severe. So, there is a difficult problem of axially aligning all the lens elements with high accuracy.
Even in another Patent, U.S. Pat. No. 4,999,007, a zoom lens with a smaller number of lens elements is proposed. Particularly for the first and second embodiments in this patent, a range of not less than 3 is realized, but the number of constituent lenses in the first lens unit is as few as 1 or 2. Accordingly, the aberrations the first lens unit produces, including chromatic aberrations, are not corrected well enough. Also, the aspherical first lens of the first embodiment has so unfavorable a form as to lessen the ease with which molding techniques are used. Concretely speaking, the paraxial and marginal zones largely differ in thickness. Therefore, as it takes form in the mold, the lens is hardly detached from the mold. In the second embodiment, the above-described drawback is small, but the angle of view is narrow, suggesting that the design does not aim at extending the wide angle end toward sufficiently shorter focal lengths. In addition, the entire lens system has a long total length and is not suited to improve the compactness of the camera.